Helium plays a major role in the pulsed atmospheric pressure operation of a transversely excited CO2 laser, commonly termed as TEA CO2 laser, mainly in stabilising the discharge of the laser. Helium, with its very low electron affinity, facilitates the occurrence of an arc free discharge at atmospheric pressure and hence has been indispensable in the conventional operation of TEA CO2 lasers. However, helium is an expensive and scarce gas and the use of helium in TEA lasers substantially increases the production as well as operational cost of such lasers. Several attempts have been made and number of special techniques have been employed in the past to obtain helium free operation of CO2 lasers.
Such methods and their limitations are briefly described below    1. Low pressure CO2 laser: There exist many reports on helium free low pressure TE CO2 lasers or helium free CW operation of low pressure CO2 lasers. In these systems the low operating pressure intrinsically ensures glow mode operation of the discharge in absence of helium.
By virtue of their low pressure operation, although they can be operated in CW mode, the maximum coherent power that can be obtained from such systems is at least three orders of magnitude lower than the TEA systems.    2. Rapid discharge technique: This approach takes advantage of a very rapid discharge (few tens of nsec as against hundreds of nsec in a conventional operation) to realise helium free operation, as the glow to arc transition in absence of helium is very fast [P. E. Dyer and B. L. Tait, Appl Phys Lett 41, 506 (1982)., P. E. Dyer and B. L. Tait, J. Phys E:Sci Instrum 16, 467 (1983)., M. Trtica, P. Vujkcvic Cvijin, and I. Mendas, Opt Quant Electron 16, 511 (1984)]. Thus discharge extinguishes before arcing can set in.
Helium free operation by rapid discharge technique can be effected only in specially designed mini laser systems that inherently offer low discharge loop inductance. Such operation, therefore, restricts the active volume and hence the maximum obtainable energy output from the system. Rapid excitation invariably results in the emission of optical pulses with short duration and high peak power. Conventional long pulse operation is therefore not possible by this method.    3. Seeding the laser gas mixture with Low Ionisation Potential (LIP) additives: In the absence of helium the electrons in the discharge are lost largely by negative ion attachment processes giving rise to the formation of an arc discharge. The addition of LIP hydrocarbons increases the primary photoelectron density thereby compensating the loss of electrons in absence of helium leading to arc free operation [S. Marchetti, R. Simili, and M. Giorgi, J. de Physique 48, C7-51 (1987)].            The LIP additives seeded in the laser gas mixture undergo dissociation in an electric discharge, which tend to settle on the optics, electrodes, and the internal surface of the laser head degrading rapidly the performance of the laser.            4. Preconditioning the inter-electrode volume by electrons from an external source: Loss of electrons in absence of helium can be overcome by deluging the active volume with electrons produced externally, as in case of an electron beam controlled CO2 laser, resulting in arc free operation [U.S. Pat. No. 4,264,868]. The US patent referred to describes a high power output CO2 gas laser amplifier having a number of sections, each comprising a plurality of annular pumping chambers spaced about the circumference of a vacuum chamber containing a cold cathode, gridded electron gun. The electron beam from the electron gun ionizes the gas lasing medium in the sections. An input laser beam is split into a plurality of annular beams, each passing through the sections comprising one pumping chamber. In this system thus, helium-free operation of TEA CO2 lasers calls for an external source of electrons thereby making the system more complicated, expensive and bulky. In addition, this is achieved at the expense of the wall plug efficiency. U.S. Pat. No. 4,264,868 relates to the generation of hundreds of kJ of focusable energy contained in a pulse of duration of 1 nsec or less. The discharge stability at 1800 torr was obtained by making use of an externally generated electron beam that provided the required ionisation in a laser mixture consisting of the molecular gases alone. As stated in this US patent itself, this system is meant for a specific application. Conventional operation based on this method not only makes the system bulky but also less efficient.